Optical semiconductor device, laser chip and laser module

ABSTRACT

An optical semiconductor device has a semiconductor substrate, an optical semiconductor region and a heater. The optical semiconductor region is provided on the semiconductor substrate and has a width smaller than that of the semiconductor substrate. The heater is provided on the optical semiconductor region. The optical semiconductor region has a cladding region, an optical waveguide layer and a low thermal conductivity layer. The optical waveguide layer is provided in the cladding region and has a refractive index higher than that of the cladding region. The low thermal conductivity layer is provided between the optical waveguide layer and the semiconductor substrate and has a thermal conductivity lower than that of the cladding region.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. application Ser.No. 11/727,496, filed Mar. 27, 2007, based upon and claims the benefitof priority of the prior Japanese Patent Application No. 2006-096075,filed Mar. 30, 2006, the entire contents of which are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to an optical semiconductor device, alaser chip and a laser module.

2. Description of the Related Art

Generally, a wavelength-tunable semiconductor laser has a gain for alaser emission and can select a wavelength of the laser. There are somemethods of selecting a wavelength. For example, the methods include amethod of changing a resonant wavelength of loss or gain by changing arefractive index or angle of a diffractive grating or an etalon providedin a laser cavity. And the methods include a method of changing aresonant wavelength of the laser cavity by changing an optical length inthe laser cavity (refractive index or a physical length of the lasercavity).

The method of changing the refractive index has an advantage inreliability or manufacturing cost, because a mechanical operatingportion is not necessary being different from the method of changing theangle or length. The refractive index changing method includes changinga temperature of an optical waveguide, changing a carrier density of theoptical waveguide by providing a current, and so on. A semiconductorlaser having a Sampled Grating Distributed Reflector (SG-DR) is supposedas a wavelength tunable laser that changes a temperature of an opticalwaveguide, the SG-DR having a wavelength selection function.

In the semiconductor laser, a reflection spectrum of a plurality ofSG-DR regions (reflection region) is controlled, a wavelength isselected with a vernier effect. That is, this semiconductor laser emitsa laser light at a wavelength where reflection peaks of two SG-DRregions are overlapped with each other. It is therefore possible tocontrol the lasing wavelength by controlling each of the reflectionpeaks of the SG-DR regions.

Generally, a heater is provided on a surface of the SG-DR region. Thetemperature of the optical waveguide of the SG-DR region is changed whenthe heater generates heat. And the refractive index of the SG-DR regionchanges. It is therefore possible to control the reflection peakwavelength of the SG-DR region by controlling the heating value of theheater. The heater needs an electrical power when generating heat. Andso, Japanese Patent Application Publication No. 9-92934 discloses amethod of controlling a refractive index of a reflection region byproviding an electrical power to a heater.

Here, a wavelength controllable range of the reflection peak isproportional to an amount of refractive index change of the opticalwaveguide, an amount of temperature change of the optical waveguide. Itis necessary to enlarge the electrical power to be provided to theheater in order to enlarge the wavelength controllable range. However,it is preferable that the semiconductor laser is controlled with lowelectrical power. It is therefore difficult for the heater to generatesufficient heat with the preferable control power of the semiconductorlaser.

SUMMARY OF THE INVENTION

The present invention provides an optical semiconductor device, a laserchip and a laser module in which an optical waveguide is effectivelyheated with heat generated in a heater.

According to an aspect of the present invention, preferably, there isprovided an optical semiconductor device including a semiconductorsubstrate, an optical semiconductor region and a heater. The opticalsemiconductor region is provided on the semiconductor substrate and hasa width smaller than that of the semiconductor substrate. The heater isprovided on the optical semiconductor region. The optical semiconductorregion has a cladding region, an optical waveguide layer and a lowthermal conductivity layer. The optical waveguide layer is provided inthe cladding region and has a refractive index higher than that of thecladding region. The low thermal conductivity layer is provided betweenthe optical waveguide layer and the semiconductor substrate and has athermal conductivity lower than that of the cladding region.

With the above-mentioned configuration, the heat generated by the heateris provided to the cladding region. The thermal resistance between theoptical waveguide layer and the semiconductor substrate is enlarged,because the low thermal conductivity layer is provided. It is thuspossible to control the temperature of the optical waveguide layereffectively with the heat generated by the heater. It is thereforepossible to effectively control the lasing wavelength of a semiconductorlaser having the optical semiconductor device.

According to another aspect of the present invention, preferably, thereis provided a semiconductor laser chip including an opticalsemiconductor device and a gain region. The optical semiconductor devicehas a semiconductor substrate, an optical semiconductor region and aheater. The optical semiconductor region is provided on thesemiconductor substrate and has a width smaller than that of thesemiconductor substrate. The heater is provided on the opticalsemiconductor region. The optical semiconductor region has a claddingregion, an optical waveguide layer and a low thermal conductivity layer.The optical waveguide layer is provided in the cladding region and has arefractive index higher than that of the cladding region. The lowthermal conductivity layer is provided between the optical waveguidelayer and the semiconductor substrate and has a thermal conductivitylower than that of the cladding region. The gain region is provided onthe semiconductor substrate and is optically coupled to the opticalwaveguide layer.

With the above-mentioned configuration, the heat generated by the heateris provided to the cladding region. The thermal resistance between theoptical waveguide layer and the semiconductor substrate is enlarged,because the low thermal conductivity layer is provided. It is thuspossible to control the temperature of the optical waveguide layereffectively with the heat generated by the heater. It is thereforepossible to control the lasing wavelength of the semiconductor laserchip effectively.

According to another aspect of the present invention, preferably, thereis provided a laser module including a semiconductor laser chip and atemperature control device. The semiconductor laser chip has an opticalsemiconductor device and a gain region. The optical semiconductor devicehas a semiconductor substrate, an optical semiconductor region and aheater. The optical semiconductor region is provided on thesemiconductor substrate and has a width smaller than that of thesemiconductor substrate. The heater is provided on the opticalsemiconductor region. The optical semiconductor region has a claddingregion, an optical waveguide layer and a low thermal conductivity layer.The optical waveguide layer is provided in the cladding region and has arefractive index higher than that of the cladding region. The lowthermal conductivity layer is provided between the optical waveguidelayer and the semiconductor substrate and has a thermal conductivitylower than that of the cladding region. The gain region is provided onthe semiconductor substrate and is optically coupled to the opticalwaveguide layer. At least a part of the semiconductor laser chip isarranged on the temperature control device. The temperature controldevice controls the temperature of at least a part of the semiconductorlaser chip.

With the above-mentioned configuration, the temperature control devicecan control the temperature of the semiconductor laser chip. And it ispossible to control the lasing wavelength of the semiconductor laserchip. And the heat generated by the heater is provided to the claddingregion. The thermal resistance between the optical waveguide layer andthe semiconductor substrate is enlarged, because the low thermalconductivity layer is provided. It is thus possible to control thetemperature of the optical waveguide layer effectively with the heatgenerated by the heater. It is therefore possible to control the lasingwavelength of the laser module effectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described indetail with reference to the following drawings, wherein:

FIG. 1 illustrates a cross sectional view of a distributed reflector inaccordance with a first embodiment of the present invention;

FIG. 2A and FIG. 2B illustrate a relationship between composition and aproperty of In_(1-x)Ga_(x)As_(y)P_(1-y);

FIG. 3 illustrates a relationship between composition and a refractiveindex of In_(1-x-y)Al_(x)Ga_(y)As;

FIG. 4 illustrates a relationship between composition and a refractiveindex of In_(1-x)Al_(x)As_(y)P_(1-y);

FIG. 5 illustrates a perspective view of a semiconductor laser chip inaccordance with a second embodiment of the present invention;

FIG. 6A illustrates a top view of a semiconductor laser chip;

FIG. 6B illustrates a cross sectional view taken along a line A-A ofFIG. 6A;

FIG. 7 illustrates an overall structure of a laser module in accordancewith a third embodiment of the present invention; and

FIG. 8 illustrates an overall structure of a laser module in accordancewith a fourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to the accompanyingdrawings, of embodiments of the present invention.

First Embodiment

In a first embodiment, a description will be given of a distributedreflector as an example of an optical semiconductor device in accordancewith the present invention. FIG. 1 illustrates a cross sectional view ofa distributed reflector 100 in accordance with the first embodiment. Asshown in FIG. 1, the distributed reflector 100 has a structure in whichan optical semiconductor region 102 is provided on a center area of asemiconductor substrate 101, the optical semiconductor region 102 havinga mesa shape and having a width smaller than that of the semiconductorsubstrate 101. That is, the optical semiconductor region 102 isprojecting from the center area of the semiconductor substrate 101.

The semiconductor substrate 101 is, for example, composed of InP. Aninsulating layer (not shown) is provided on a top face and a side faceof the optical semiconductor region 102 and on an exposed top face ofthe semiconductor substrate 101. The insulating layer acts as apassivation layer. The insulating layer is, for example, composed of aninsulator such as SiO₂. A heater 103 is provided on the opticalsemiconductor region 102. The optical semiconductor region 102 has astructure in which an optical waveguide layer 105 is formed in a centerarea of a cladding region 104. The cladding region 104 has a low thermalconductivity layer 106 between the optical waveguide layer 105 and thesemiconductor substrate 101.

A distance from the top face of the optical semiconductor region 102 toa top face of the optical waveguide layer 105 is, for example,approximately 3 μm. A distance from a lower face of the opticalwaveguide layer 105 to the low thermal conductivity layer 106 is, forexample, 3 μm. A thickness of the low thermal conductivity layer 106 is,for example, 1 μm. A height of the optical semiconductor region 102 is,for example, 20 μm. The cladding region 104 is, for example, composed ofInP. The optical waveguide layer 105 is, for example, composed of amaterial having an absorption edge wavelength at shorter wavelengthsside compared to a lasing wavelength. The optical waveguide layer 105is, for example, composed of InGaAsP crystal.

The low thermal conductivity layer 106 is composed of an alloysemiconductor crystal. The thermal conductivity of the low thermalconductivity layer 106 is therefore lower than that of the claddingregion 104. In the embodiment, the low thermal conductivity layer 106 iscomposed of In_(1-x)Ga_(x)As_(y)P_(1-y), In_(1-x-y)Al_(x)Ga_(y)As orIn_(1-x)Al_(x)As_(y)P_(1-y). The thickness of the low thermalconductivity layer 106 is, for example, 1 μm. The low thermalconductivity layer 106 may be composed of Al_(x)Ga_(1-x)As alloysemiconductor crystal or Ga_(1-x)In_(x)N_(y)As_(1-y) alloy semiconductorcrystal, when GaAs is used for the semiconductor substrate 101. Theheater 103 is composed of such as NiCr and generates heat according toan electrical power provided thereto.

The heat generated by the heater 103 is provided to the cladding region104. The thermal resistance between the optical waveguide layer 105 andthe semiconductor substrate 101 is enlarged because the low thermalconductivity layer 106 is provided in the embodiment. It is thuspossible to control the temperature of the optical waveguide layer 105effectively with the heat generated by the heater 103. It is thereforepossible to control the lasing wavelength of a semiconductor laserhaving the distributed reflector 100. Here, the low thermal conductivitylayer 106 is provided on overall in a width direction of the opticalsemiconductor region 102.

It is preferable that the refractive index of the low thermalconductivity layer 106 is lower, because in this case it is restrainthat a light propagating in the optical waveguide layer 105 is leaked tothe low thermal conductivity layer 106. A description will be given of apreferable composition range of In_(1-x)Ga_(x)As_(y)P_(1-y),In_(1-x-y)Al_(x)Ga_(y)As and In_(1-x)Al_(x)As_(y)P_(1-y).

FIG. 2A and FIG. 2B illustrate a relationship between the composition ofIn_(1-x)Ga_(x)As_(y)P_(1-y) substantially lattice matched to an InPsubstrate as the semiconductor substrate 101 and the property thereof.FIG. 2A illustrates a relationship between the composition ofIn_(1-x)Ga_(x)As_(y)P_(1-y) and the thermal conductivity thereof. FIG.2B illustrates a relationship between the composition ofIn_(1-x)Ga_(x)As_(y)P_(1-y) and the refractive index thereof. Thehorizontal axis of FIG. 2A and FIG. 2B indicates the As composition y(atm %). The vertical axis of FIG. 2A indicates the thermal conductivityof In_(1-x)Ga_(x)As_(y)P_(1-y). The vertical axis of FIG. 2B illustratesa ratio of a differential between the refractive index ofIn_(1-x)Ga_(x)As_(y)P_(1-y) and the refractive index of InP against therefractive index of InP. The ratio is referred to as a refractive indexdifferential D1 (%).

As shown in FIG. 2A, the thermal conductivity ofIn_(1-x)Ga_(x)As_(y)P_(1-y) gets lower as the As composition y getshigher. This is because a freedom degree of atom arrangement of thesemiconductor is enlarged. The thermal conductivity ofIn_(1-x)Ga_(x)As_(y)P_(1-y) is lower than that of InP, when at least theAs composition y>0. It is preferable that the thermal conductivity of amaterial used for the low thermal conductivity layer 106 is less than1/10 of that of the cladding region 104. And it is preferable that theAs composition y is more than 15%.

On the other hand, as shown in FIG. 2B, the refractive index ofIn_(1-x)Ga_(x)As_(y)P_(1-y) gets higher as the As composition y getshigher. It is possible to restrain a leakage of a light propagating inthe optical waveguide layer 105 to the low thermal conductivity layer106, when the refractive index differential D1 is under 6%. It istherefore preferable that the As composition y is less than 55%.Accordingly, it is preferable that the As composition y is above 0 andless than 55 atm % and it is more preferable that the As composition yis more than 15 atm % and less than 55 atm %, whenIn_(1-x)Ga_(x)As_(y)P_(1-y) is used for the low thermal conductivitylayer 106.

Next, a description will be given of composition ofIn_(1-x-y)Al_(x)Ga_(y)As. The thermal conductivity ofIn_(1-x-y)Al_(x)Ga_(y)As is less than 1/10 of that of InP, whenIn_(1-x-y)Al_(x)Ga_(y)As is substantially lattice matched to the InPsubstrate. It is therefore possible to use In_(1-x-y)Al_(x)Ga_(y)As asthe low thermal conductivity layer 106. Next, a description will begiven of a relationship between the composition ofIn_(1-x-y)Al_(x)Ga_(y)As and the refractive index thereof.

FIG. 3 illustrates a relationship between the composition ofIn_(1-x-y)Al_(x)Ga_(y)As substantially lattice matched to the InPsubstrate and the refractive index thereof. The horizontal axis of FIG.3 indicates the Al composition x (atm %). The vertical axis of FIG. 3indicates a ratio of a differential between the refractive index ofIn_(1-x-y)Al_(x)Ga_(y)As and the refractive index of InP against therefractive index of InP. The ratio is referred to as a refractive indexdifferential D2 (%). As shown in FIG. 3, the refractive indexdifferential D2 gets lower as the Al composition x gets higher. Therefractive index differential D2 is under 6%, when the Al composition xis more than 26%. It is therefore preferable that the Al composition xis more than 26%.

Next, a description will be given of composition ofIn_(1-x)Al_(x)As_(y)P_(1-y). The thermal conductivity ofIn_(1-x)Al_(x)As_(y)P_(1-y) is lower than that of InP in a case wherethe In_(1-x)Al_(x)As_(y)P_(1-y) is substantially lattice matched to theInP substrate, when the Al composition x is above 0 or the Ascomposition y is above 0. It is therefore possible to useIn_(1-x)Al_(x)As_(y)P_(1-y) for the low thermal conductivity layer 106,at least when the Al composition x is above 0 or the As composition y isabove 0. Next, a description will be given of a relationship between thecomposition and the refractive index of In_(1-x)Al_(x)As_(y)P_(1-y).

FIG. 4 illustrates a relationship between the composition ofIn_(1-x)Al_(x)As_(y)P_(1-y) substantially lattice matched to the InPsubstrate and the refractive index thereof. The horizontal axis of FIG.4 indicates the As composition y (atm %). The vertical axis of FIG. 4indicates a ratio of a differential between the refractive index ofIn_(1-x)Al_(x)As_(y)P_(1-y) and the refractive index of InP against therefractive index of InP. The ratio is referred to as a refractive indexdifferential D3 (%). As shown in FIG. 4, the refractive indexdifferential D3 gets higher as the As composition y gets higher.However, the refractive index differential D3 does not surpass 6%regardless of the As composition y. It is therefore possible to useIn_(1-x)Al_(x)As_(y)P_(1-y) for the low thermal conductivity layer 106regardless of the As composition y. Here, a range where a semiconductormixed crystal is substantially lattice matched to the InP substratemeans a range where the alloy semiconductor crystal on the InP substratedoes not cause strain relaxation.

Second Embodiment

A description will be given of a semiconductor laser chip 200 inaccordance with a second embodiment of the present invention. FIG. 5illustrates a perspective view of the semiconductor laser chip 200. FIG.6A illustrates a top view of the semiconductor laser chip 200. FIG. 6Billustrates a cross sectional view taken along a line A-A of FIG. 6A. Adescription will be given of the semiconductor laser chip 200 withreference to FIG. 5, FIG. 6A and FIG. 6B.

As shown in FIG. 5, FIG. 6A and FIG. 6B, the semiconductor laser chip200 has a structure in which a Sampled Grating Distributed Reflector(SG-DR) region α, a Sampled Grating Distributed Feedback (SG-DFB) regionβ and a Power Control (PC) region γ are coupled in order.

The SG-DR region α has a structure in which a lower cladding layer 5 a,an optical waveguide layer 3, an upper cladding layer 5 b and aninsulating layer 6 are laminated on a semiconductor substrate 1 in orderand a heater 9, a power electrode 10 and a ground electrode 11 arelaminated on the insulating layer 6. The SG-DFB region β has a structurein which the lower cladding layer 5 a, an optical waveguide layer 4, theupper cladding layer 5 b, a contact layer 7 and an electrode 8 arelaminated on the semiconductor substrate 1 in order. The PC region γ hasa structure in which the lower cladding layer 5 a, an optical waveguidelayer 12, the upper cladding layer 5 b, a contact layer 13 and anelectrode 14 are laminated on the semiconductor substrate 1 in order.The semiconductor substrate 1, the lower cladding layer 5 a and theupper cladding layer 5 b of the SG-DR region α, the SG-DFB region β andthe PC region γ are a single layer formed as a unit respectively. A lowthermal conductivity layer 51 is formed from a low reflecting coating 15to a low reflecting coating 16 in the lower cladding layer 5 a. Theoptical waveguide layers 3, 4 and 12 are formed on a same plane and areoptically coupled.

The low reflecting coating 15 is formed on end facet of thesemiconductor substrate 1, the optical waveguide layer 3, the lowercladding layer 5 a and the upper cladding layer 5 b at the SG-DR regionα side. On the other hand, the low reflecting coating 16 is formed onend facet of the semiconductor substrate 1, the optical waveguide layer12, the lower cladding layer 5 a and the upper cladding layer 5 b at thePC region γ side. Diffractive gratings 2 are formed at a given intervalin the optical waveguide layers 3 and 4. The sampled grating is thusformed. The insulating layer 6 is further formed between the electrode 8and the electrode 14.

The semiconductor substrate 1 is, for example, composed of InP. Theoptical waveguide layer 3 is, for example, composed of InGaAsP crystalhaving an absorption edge wavelength at shorter wavelengths sidecompared to the lasing wavelength. PL wavelength of the opticalwaveguide layer 3 is approximately 1.3 μm. The optical waveguide layer 4is, for example, an active layer composed of InGaAsP crystal amplifyinga light of a desirable wavelength of a laser emission. The PL wavelengthof the optical waveguide layer 4 is approximately 1.57 μm. The opticalwaveguide layer 12 is, for example, composed of InGaAsP crystal forchanging the output of the emitted light by absorbing or amplifying alight. The PL wavelength of the optical waveguide layer 12 isapproximately 1.57 μm.

SG-DR optical waveguide segments are formed in the optical waveguidelayer 3. Three SG-DR optical waveguide segments are formed in theoptical waveguide layer 3 in the embodiment. Here, the SG-DR opticalwaveguide segment is a region in which one region having the diffractivegrating 2 and one space region not having the diffractive grating 2 arecombined in the optical waveguide layer 3.

The lower cladding layer 5 a and the upper cladding layer 5 b arecomposed of InP and confine a laser light traveling in the opticalwaveguide layers 3, 4 and 12. The lower cladding layer 5 a has the lowthermal conductivity layer 51. The low thermal conductivity layer 51 iscomposed of a material having a thermal conductivity lower than that ofthe lower cladding layer 5 a. The low thermal conductivity layer 51 is,for example, composed of the same material of the low thermalconductivity layer 106. The contact layers 7 and 13 are composed ofInGaAsP crystal. The insulating layer 6 is a passivation film composedof an insulator such as SiN, SiO₂. The low reflecting coatings 15 and 16are, for example, composed of a dielectric film including MgF₂ and TiON.The reflectivity of the low reflecting coatings 15 and 16 are, forexample, less than 0.3%.

The heater 9 is composed of such as NiCr. The heater 9 is formed on theinsulating layer 6. The power electrode 10 and the ground electrode 11are coupled to the heater 9. The power electrode 10, the groundelectrode 11, the electrode 8 and the electrode 14 are composed of aconductive material such as Au. As shown in FIG. 5, a mesa groove 21 isformed from both sides of the heater 9 to the semiconductor substrate 1passing through both sides of the optical waveguide layer 3. The mesagroove 21 is formed to be parallel to the optical waveguide layer 3. Inthe embodiment, a mesa semiconductor region 20 corresponds to theoptical semiconductor region 102 of the first embodiment. The mesasemiconductor region 20 is demarcated with the mesa groove 21 and hasthe optical waveguide layer 3.

Next, a description will be given of an operation of the semiconductorlaser chip 200. At first, a current is provided to the electrode 8. Anda light is generated in the optical waveguide layer 4. The lightpropagates in the optical waveguide layers 3 and 4, and is reflected andamplified repeatedly. Then, it causes lasing oscillation. A part of thelaser light is amplified or absorbed in the optical waveguide layer 12and is emitted through the low reflecting coating 16. It is possible tocontrol the gain or the absorptance of the optical waveguide layer 12with the current provided to the electrode 14. The output of the emittedlight is kept constant when a predetermined current is provided to theelectrode 14.

When an electrical power is provided to the heater 9, the temperature ofeach SG-DR segment is controlled according to the electrical power.Therefore the refractive index of the SG-DR segment changes. And areflection peak wavelength of the optical waveguide layer 3 changes. Itis possible to control the lasing wavelength of the semiconductor laserchip 200 by controlling the electrical power to be provided to theheater 9.

The thermal resistance between the optical waveguide layer 3 and thesemiconductor substrate 1 is enlarged, because the low thermalconductivity layer 51 is provided in the lower cladding layer 5 a in theembodiment. It is thus possible to control the temperature of theoptical waveguide layer 3 effectively with the heat generated by theheater 9. It is therefore possible to control the lasing wavelength ofthe semiconductor laser chip 200 effectively. The low thermalconductivity layer 51 may be formed only in the SG-DR region α.

Third Embodiment

A description will be given of a laser module 300 in accordance with athird embodiment of the present invention. FIG. 7 illustrates an overallstructure of the laser module 300. As shown in FIG. 7, the laser module300 has a structure in which a semiconductor laser chip 302 is providedon a temperature control device 301. The semiconductor laser chip 302 isthe same as the semiconductor laser chip 200 of the second embodiment.

The temperature control device 301 controls the temperature of thesemiconductor laser chip 302. The temperature control device 301 cancontrol the reflection peak wavelength of the optical waveguide layer 4.And the laser module 300 can control the lasing wavelength withtemperature control of the optical waveguide layer 3 by the heater 9 andwith the temperature control of the optical waveguide layer 4 by thetemperature control device 301.

Fourth Embodiment

A description will be given of a laser module 300 a in accordance with afourth embodiment of the present invention. FIG. 8 illustrates anoverall structure of the laser module 300 a. The laser module 300 adiffers from the laser module 300 shown in FIG. 7 in a position wherethe semiconductor laser chip 302 is arranged on the temperature controldevice 301. As shown in FIG. 8, the PC region γ and the SG-DFB region βare arranged on the temperature control device 301. The SG-DR region αis not arranged on the temperature control device 301.

In this case, the temperature control device 301 controls thetemperature of the SG-DFB region β. And the temperature control device301 can control the reflection peak wavelength of the optical waveguidelayer 4. Accordingly, the laser module 300 a in accordance with theembodiment can control the lasing wavelength with the temperaturecontrol of the optical waveguide layer 3 by the heater 9 and with thetemperature control of the optical waveguide layer 4 by the temperaturecontrol device 301. Therefore, the SG-DR region α may not be arranged onthe temperature control device 301. In addition, it is easy for thetemperature control device 301 to control the temperature of the opticalwaveguide layer 4, when the low thermal conductivity layer 51 is notprovided in the SG-DFB region β.

While the above description constitutes the preferred embodiments of thepresent invention, it will be appreciated that the invention issusceptible of modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

The present invention is based on Japanese Patent Application No.2006-096075 filed on Mar. 30, 2006, the entire disclosure of which ishereby incorporated by reference.

What is claimed is:
 1. An optical semiconductor device comprising: asemiconductor substrate; an optical semiconductor region that isprovided on the semiconductor substrate and has a width smaller thanthat of the semiconductor substrate; and a heater that is provided onthe optical semiconductor region, the optical semiconductor regionhaving a cladding region, an optical waveguide layer and a low thermalconductivity layer, the optical waveguide layer being provided in thecladding region and having a refractive index higher than that of thecladding region, the low thermal conductivity layer being providedbetween the optical waveguide layer and the semiconductor substrate andhaving a thermal conductivity lower than that of the cladding region,the low thermal conductivity layer being composed of a material having arefractive index lower than that of the optical waveguide layer, adifference between a refractive index of the low thermal conductivitylayer and a refractive index of the cladding region being under 6%, thecladding region being composed of InP, the low thermal conductivitylayer being composed of InAlGaAs or InAlAsP.
 2. The opticalsemiconductor device as claimed in claim 1, wherein the low the thermalconductivity layer is provided on overall in a width direction of theoptical semiconductor region.
 3. The optical semiconductor device asclaimed in claim 1, wherein the low thermal conductivity layer iscomposed of a material having a thermal conductivity lower than 1/10 ofthat of the cladding region.
 4. The optical semiconductor device asclaimed in claim 1, wherein the low thermal conductivity layer isInAlGaAs semiconductor crystal, is substantially lattice matched to theInP and has Al so that Al composition against InGa is more than 26%. 5.The optical semiconductor device as claimed in claim 1, furthercomprising a second cladding region provided between the opticalwaveguide layer and the low thermal conductivity layer.